refacto: add RenderingEngine::cleanupMeshCache

This permits to prevent client to own the mesh cache cleanup logic. It's better in RenderingEngine

src/client/renderingengine.h

@@ -56,6 +56,7 @@ class RenderingEngine
 	bool setWindowIcon();
 	bool setXorgWindowIconFromPath(const std::string &icon_file);
 	static bool print_video_modes();
+	void cleanupMeshCache();
 
 	static RenderingEngine *get_instance() { return s_singleton; }